As is known, by means of flow measuring devices containing a magneto-inductive measuring transducer, the volume flow of an electrically conductive fluid flowing through a measuring tube of such measuring transducer in a flow direction can be measured. To this end, there is produced at the measuring transducer, by means of a magnetic circuit arrangement electrically connected to an exciter-electronics of the flow measuring device, a magnetic field of highest possible density, passing through the fluid within a measurement volume at least sectionally perpendicularly to the flow direction and closing on itself essentially outside of the fluid. The measuring tube is, therefore, usually made of non-ferromagnetic material, in order that the magnetic field not be unfavorably influenced during measurement. As a result of movement of free charge carriers of the fluid in the magnetic field, there is produced in the measurement volume, according to the magneto-hydrodynamic principle, an electrical field, which extends perpendicularly to the magnetic field and perpendicularly to the flow direction of the fluid. By means of at least two measuring electrodes arranged spaced from one another in the direction of the electrical field and by means of an evaluating electronics of the flow measuring device connected to these electrodes, thus, an electric voltage induced in the fluid is measurable. This voltage is a measure for the volume flow. The measuring transducer is so constructed, that the induced electrical field closes on itself outside of the fluid practically exclusively via the evaluating electronics connected to the measuring electrodes. For tapping the induced voltage, for example, fluid-contacting, galvanic, or fluid non-contacting, capacitive, measuring electrodes can serve.
Due to the required high mechanical stability for such measuring tubes, these are composed, preferably, of an outer, especially metal, support tube of predeterminable strength and size, which is coated internally with an electrically non-conductive, insulating material of predeterminable thickness, the so-called liner. For example, described in WO-A 05/057141, WO-A 04/072590, US-A 2005/0000300, U.S. Pat. No. 6,595,069, U.S. Pat. No. 5,280,727, U.S. Pat. No. 4,679,442, U.S. Pat. No. 4,253,340, U.S. Pat. No. 3,213,685 or JP-Y 53-51 181, in each case, are magneto-inductive measuring transducers employing a measuring tube joinable pressure-tightly into a pipeline. The measuring tube has an inlet-side, first end and an outlet-side, second end and is composed of a, most often, non-ferromagnetic support tube as an outer jacket, and a tubular liner accommodated in a lumen of the support tube, composed of an insulating material and serving for guiding the flowing fluid insulated from the support tube. The liner thus serves to guide a fluid e.g. chemically isolated from the support tube. In the case of support tubes of higher electrical conductivity, especially in the case of metal support tubes, the liner, moreover, serves as electrical insulation between the support tube and the fluid, for preventing a short-circuiting of the electrical field via the support tube. By an appropriate design of the support tube, thus implementable is a matching of the strength of the measuring tube to the mechanical loadings present in the respective instance of use, combined with a matching of the liner of the measuring tube to the chemical, especially hygienic, requirements for the respective instance of use. For manufacture of the liner, often applied are the injection-molding or transfer-molding methods. It is, however, also usual, to use a completely prefabricated liner in the support tube. Thus, JP-A 59-137 822 discloses a method, in which the liner is formed by softening a film of synthetic material, or plastic. Usually embedded in the liner, made, most often, of a thermoplastic or thermosetting, synthetic material, is an open-pored, especially metal, support body for stabilizing the liner, such practice being disclosed, for example, also in EP-A 36 513, EP-A 581 017, JP-Y 53-51 181, JP-A 59-137 822, U.S. Pat. No. 6,595,069, U.S. Pat. No. 5,664,315, U.S. Pat. No. 5,280,727 or U.S. Pat. No. 4,329,879. The support bodies are, in each case, accommodated aligned in the lumen of the measuring tube and completely encased by the insulating material, at least on the fluid contacting, inside.
Especially the flow meter disclosed in U.S. Pat. No. 4,253,340 or in WO-A 05/057141 includes, further, a support frame for holding the measuring tube and for holding an electronics housing mechanically connected with the measuring transducer. The electronics housing serves to accommodate the above-mentioned exciter- and evaluating-electronics near to the measuring transducer and to protect it largely from environmental influences. Measuring tube and support frame are, in such case, affixed to one another only on the inlet side and on the outlet side, in each case, over a comparatively short, connection region. Flow measuring devices of the type shown in U.S. Pat. No. 4,253,340 are distinguished, among others, by the fact that they can be constructed very compactly.
For guiding and coupling the magnetic field into the measurement volume, the magnetic circuit arrangement usually includes two coil cores of magnetically conductive material, which are arranged on a periphery of the measuring tube, especially diametrally spaced from one another and having, in each case, a free, terminal face, arranged especially as mirror images relative to one another. The magnetic field is so coupled into the coil cores by means of a coil arrangement connected to the exciter-electronics, that it passes through the fluid flowing between both end faces, at least sectionally, perpendicularly to the flow direction. On its end distal to the end face, each of the coil cores is usually additionally magnetically coupled with at least one, one- or multi-piece, separate element of magnetically conductive material providing back-closure for the field. The field-conducting element extends peripherally to the measuring tube and guides the magnetic field around the outside of measuring tube. In the case of a one-piece, peripheral, back-closing element, such can, in such case, be connected, in each case, with the respective coil core end, for example, by means of screw- and/or, as proposed in WO-A 04/072590, by means of clamp-connections.
Disadvantageous in the case of such construction is, on the one hand, the comparatively large number of individual parts for a single measuring transducer, this working against assembly in as short a time as possible and against simultaneously required, high product quality. On the other hand, it is, in the case of measuring transducers of the described kind, of special interest, to position the field coils precisely, not only in the case of assembly of the measuring transducer, but also to assure with certainty, that their assembled position remains, as much as possible, unchanged, within very narrow limits, for ensuring accuracy of measurement over the entire operating time of the respective measuring transducer. These goals cannot, however, be assured without special measures when using coil cores composed of separate parts and, on occasion, also equally separate, pole shoes. Such special measures would involve considerable extra material- and assembly-complexity in the form of, for example, special abutments or other, supplemental, interlocking mechanisms for the field coils, coil cores, and/or pole shoes.
It is, therefore, an object of the invention to improve measuring transducers of the described kind, such that the number of individual parts required for their manufacture can be reduced in comparison to conventional magneto-inductive measuring transducers. Moreover, the magnetic field system should also be as easy as possible to assemble and, additionally, exhibit a high, long-term stability.